Spinning reserve with inlet throttling and compressor recirculation



July 3, 1968 R. L. HENDRICKSON 3,394,265

SPINNING RESERVE WITH INLET THROTTLING AND COMPRESSOR RECIRCULATIONFiled D80. 15, 19 55 FIG! FUEL l 9 Am COMBUSTOR GAS/ 3 u COMPRESSOR 1TURBINE GENERATOR \\\,5 6 REClRCULATE EXHAUST THROTTLE NORMAL AIR INTAKEFIGZ INVENTOR ROBERT L. HENDRICKSON,

BY 4J- HIS ATTORNEY.

United States Patent 3,394,265 SPINNING RESERVE WITH INLET THROTTLINGAND COMPRESSOR RECIRCULATION Robert L. Hendrickson, Scotia, N.Y.,assignor to General Electric Company, a corporation of New York FiledDec. 15, 1965, Ser. No. 513,929 Claims. (Cl. 290-2) This inventionrelates generally to the use of gas turbine-generator units as reservepower units in power generation stations. In particular, this inventionrelates to an improved standby power generation capacity using gasturbine-generators in spinning reserve.

Electric power systems require blocks of capacity to be held on standbyto protect the system against a forced outage of a main generating unit.This standby capacity must be capable of developing full load in amatter of seconds after a forced outage occurs and should therefore bewarmed up and running at normal operating speed, ready to assume a load.This running standby capacity is known as spinning reserve. The problemthat it presents is in providing this reserve capacity at minimum cost.

It has been proposed in the past to employ steam turbine generators forthis purpose, using the generator as a motor to keep the unit up tospeed and putting into the turbine only enough steam to maintain theturbine parts at temperatures approximating their operating temperaturesto avoid thermal shock when the unit is called upon to carry load. Thisis no longer practical simply because the costly reheat steam units inpresent-day powerplants cannot be economically regulated to standbyduty.

The power developed in the turbine element of a gas turbine is used intwo ways. First, it goes to drive the compressor and the remainder goesinto useful output which, for purposes of illustrating this invention,will be considered to be a generator. A generator must be driven atconstant speed as, for example, 3600 rpm, regardless of the load on thegenerator. This means that the gas turbine and hence its compressor,which is either on the same shaft or geared to the generator shaft, arerotating at full speed regardless of the power level. Withoutmodification, the compressor would be taking in the same amount of airat all power levels, absorbing a higher fraction of the turbine outputas the load is reduced. This leads to relatively high fuel consumptionat low power output. In order to reduce the flow of air through a gasturbine combustor when the turbine is running at lower power levels, itis known that a throttling of inlet air to the compressor will permit areduction of fuel consumption. It is also known that air flow throughthe combustor of a gas turbine can be reduced, and hence fuel saved atlower power levels, by extracting compressor air and recirculating it tothe compressor inlet or to a lower compressor stage, so that thecombustor receives air flow equal to the difference of air inlet flowand air recirculation flow.

When compressors are operating under certain conditions of mass flow,pressure ratio and rotational speed, they become subject to thephenomenon known as surging; that is, the fluid which normally flows apredictable path through the compressor becomes unstable causing thepressure to fall considerably. The relationship of the above parametersis such that at a given speed, surging is more likely to occur at highercompression ratios, or higher discharge pressures, and lower mass flowsthrough the compressor. It can be seen, then, that throttling byreducing mass flow, tends to cause the gas-turbine compressor to reachits surge limit. However, if the compressor discharge is partiallyrecirculated to the compressor inlet, the result is a lower compressionratio and lower compressor discharge pressure. Thus, though mass flow isdecreased by inlet throttling, pressure ratio and discharge pressure arealso decreased and surging is avoided. Therefore, with the combinedeffect of recirculation, compressor inlet can be throttled to an extentgreater than would be possible without the recirculation.

The object of this invention is to provide a means and method whereby agas turbine can be run at operating speed and temperature with minimumload, no load, or even at negative load (absorbing power from thegenerator) with a minimal fuel consumption, and be ready for duty on amoments notice.

Other objects, advantages and features of this invention will becomeapparent from the following description taken in connection with theaccompanying drawing.

Briefly stated, the essence of this invention includes a gasturbine-generator unit, connected on the line of a power generationsystem. The generator takes power from the system and thus, as a motor,is the prime mover of the unit. The gas turbine air intake is throttledand the compressor air extracted and recirculated to the intake toprovide minimum air flow to the combustor and hence minimum fuelconsumption. The recirculated compressed air provides higher airtemperature to the combustor for better combustion. The gas turbine hasthus been relieved of its mechanical load, and is being carried by thegeneratormotor. The combustor is burning a minimum of fuel with less airflow, primarily for the purpose of maintaining operating temperatures inthe turbine.

In the drawing:

FIG. 1 is a schematic representation of a gas turbinegenerator unitaccording to this invention.

FIG. 2 is a curve of turbine efficiency as a function of blade velocityand gas nozzle velocity.

Referring now to FIG. 1 for the purpose of illustrating this invention,a gas turbine-generator unit is shown comprising a compressor 1, acombustion chamber or combustor 2, a gas turbine 3 and a dynamoelectricmachine or generator 4 connected to a power system represented as 5. Inthis arrangement and under normal operating conditions, air is admittedthrough an inlet conduit 6 to the compressor 1 where it is compressedand delivered through connecting conduit 7 to the combustor 2. Fuelenters the combustor 2 through conduit 8. The air and fuel are burned incombustor 2 to produce hot gas which flows through conduit 9 to theturbine 3, exhausting through conduit 10 to atmosphere. The turbineconverts the thermal energy of the gas to mechanical energy to drive thecompressor 1 and the generator 4. Turbine 3 and compressor 1 areconnected to a common shaft 11. Turbine 3 and generator 4 are in turnconnected to a common shaft 12. Shafts 11 and 12 are connected to eachother. That is to say, turbine 3 is of the single Shaft type in whichcompressor, turbine, and external load occupy the same shaft.

The foregoing has been descriptive of a typical gas turbine-generatorunit under normal operation. It will be understood that while a singlecombustor and conduit system has been shown, there may be several ofthem arranged around the turbine shaft. Also it will be understood thatwhile the generator is shown direct-connected to its driving turbine,there may be reduction gears between the tubine and the generator.

In addition to the basic structure of a gas turbine plant as describedabove, there is included in the turbine-generator unit of this inventionan inlet throttle valve 13 in the inlet conduit 6. A recirculation line14 which includes a recirculation control valve 15 connects a highpressure section of the compressor 1 to the inlet conduit 6 at a pointbetween inlet throttle valve 13 and the inlet to compressor 1. Thoughnot shown, it is also possible to have recirculation line 14 enter thecompressor 1 at a low pressure stage in the compressor other than thecompressor inlet.

The operation of the present invention will now be described, using theabove-described normal operation as a frame of reference. The powernormally required to be developed in the turbine is not required whenused in its spinning reserve mode according to this invention. The powerordinarily required to drive the generator 4 is not required since thegenerator is being driven as a motor on the system 5. The powerordinarily required to drive the compressor is greatly reduced because(1) free air is being throttled by valve 13, and (2) hot, pressurizedair is being recirculated through conduit 14 to the compressor inlet.Air flow through the combustor is reduced by (1) the amount ofcompressor recirculation, (2) the reduced inlet pressure provided by thethrottle, and (3) the increased temperature provided by recirculatingthe hot pressurized air, thus permitting efiicient combustion with lessfuel. The higher air temperature resulting from the compressorrecirculation due to heating of the air during compression, providesimproved combustion and therefore still further fuel economy. Thecombined effects of inlet throttling and compressor recirculation are topermit a minimum of fuel to increase the turbine temperatures tosubstantially their full load or normal operating values, thus reducingor eliminating thermal shock that would otherwise result from a rapidincrease to full load temper-atures.

FIG. 2 is a curve of the engine efficiency of a pure impulse turbine. Itis not intended to define limits within this invention but rather as arepresentation of the operation of the inventon for purposes of graphicillustration. Engine or turbine efficiency is the ratio of actual workto ideal work performed by an ideal engine using the same fluid. Theindependent variable plotted on the abscissa is the variable ratio W/Vwhere W is the turbine blade tip velocity and V is the velocity of theturbine nozzle fluid. A pure impulse turbine reaches its peak engineefiiciency where the blade speed W is half the speed of the workingfluid. As can be readily visualized and as the curve shows, there is noturbine output when the blade and working fluid are moving atapproximately the same speed. The continuation of the curve where engineefficiency is negative and W/V is greater than one (blade tip movingfaster than working fluid) is representative of the case where theturbine is being driven by the generator, absorbing energy from theelectrical system instead of puting energy into it.

The presently preferred arrangement of this invention is to have theunit operating somewhere in the range labeled B on FIG. 2. This meansthat more power is being absorbed from the system and less fuel is beingburned-than in the A range. This is presently preferred because it ismore economical to draw electrical energy from the system for spinningthan to consume the higher cost gas turbine fuels.

Typical values for inlet throttling are in the approximate range of A toMs atmospheres pressure at the compressor inlet. Recirculation of thecompressor air is preferably in the range of 20-45 percent. The optimumspinning operation of a gas turbine will be a function of a particularcombination of these two variables which in turn is dependent upon thecharacteristics of individual gas turbine components and their fuels.The best combination occurs when each of the following conditions ismet:

(1) Maximum turbine temperature,

(2) Minimum compressor surge margin,

(3) Minimum combustion temperature rise,

(4) Minimum combustor air flow,

(5) Minimum fuel consumption in the combustor.

It will occur to others skilled in the art to make experiments guided bythe foregoing to arrive at optimum conditions for a particular unit.

In another modification of this invention, it is possible to use a gasturbine of two-shaft type. That is to say, that shafts 11 and 12 areseparate and the turbine is a gas turbine with a plurality of stages, ineffect, two separate turbines within the common casing, one beingconnected to and driving the compressor, and the other being-connectedto the generator. In this modification the generator drives the lowerstage turbine, but the compressor turbine is maintaining compressorspeed, requiring'at least a minimum of power and therefore more fuelconsumption as compared to the first-described system in which fuel isburned but no power derived therefrom.

Thus, it will be apparent that a gas turbine-generator unit has beendescribed which will operate at a minimum or a negative power output andat operating speed and temperature in order to be ready to assume loadand produce full power in seconds. Furthermore, it will be apparent thatthis spinning reserve capacity is produced with relative economy, bothfrom the minimum fuel burned and from the lower initial cost of a gasturbine plant.

It will occur to others of ordinary skill in the art to makemodifications of this invention which will remain within the concept andscope thereof and will not constitute patentable departure therefrom.Accordingly, it is intended that the invention be not limited by thedetails in which it has been described but that it encompasses allwithin the purview of the following claims.

What is claimed. is:

1. A heat engine operativ-ely connected to a dynamoelectric machine,said dyna moelectric machine connected to an electric power system, saidheat engine comprising an air supply conduit, an air compressor, a fuelsource, a combustor to combine fuel and air for combustion, a gasturbine into which the products of said combustion expand, means tothrottle the intake of air through the supply conduit to said heatengine, and means to extract pressurized air from said compressor andrecirculate it to a lower pressure part thereof, so that said combustorreceives air flow equal to the difference of air inlet flow and airrecirculation flow.

2. A method of operating a gas turbine-generator, which is connected toan electric power system, in standby reserve comprising:

(a) driving said turbine at a speed substantially corresponding togenerator synchronous speed using said generator as a motor,

(b) throttling compressor inlet air to reduce air flow through saidturbine,

(c) extracting compressor air from a point of relatively highertemperature and pressure,

((1) recirculating said extracted compressor air to a lower pressurepart of said compressor, further reducing air flow through said turbine.

3. The method described in claim 2 further including the step of burningfuel in the combustor of said turbine in the presence of air from saidcompressor.

4. The method according to claim 2 wherein the inlet throttling is suchas to provide A1 to /5 atmospheric pressure at the compressor inlet andwherein the recirculated air is between 20 and 45 percent of total airflow through the compressor.

5. The method according to claim 2 including the step of burning fuel inthe combustor and adjusting the extent of air throttling and extent ofair recirculation with respect to one another to provide minimum fuelconsumption at a selected turbine operating temperature. 1

No references cited.

ORIS L. RADER, Primary Examiner. G. SIMMONS, Assistant Examiner.

1. A HEAT ENGINE OPERATIVELY CONNECTED TO A DYNAMOELECTRIC MACHINE, SAIDDYNAMOELECTRIC MACHINE CONNECTED TO AN ELECTRIC POWER SYSTEM, SAID HEATENGINE COMPRISING AN AIR SUPPLY CONDUIT, AN AIR COMPRESSOR, A FUELSOURCE, A COMBUSTOR TO COMBINE FUEL AND AIR FOR COMBUSTION, A GASTURBINE INTO WHICH THE PRODUCTS OF SAID COMBUSTION EXPAND, MEANS TOTHROTTLE THE INTAKE OF AIR THROUGH THE SUPPLY CONDUIT TO SAID HEATENGINE, AND MEANS TO EXTRACT PRESSURIZED AIR FROM SAID COMPRESSOR ANDRECIRCULATE IT TO A LOWER PRESSURE PART THEREOF, SO THAT SAID COMBUSTORRECEIVES AIR FLOW EQUAL TO THE DIFFERENCE OF AIR INLET FLOW AND AIRRECIRCULATION FLOW.